Śaunaka Śikṣā 13

13

dīrghātparastvanusvāraḥ pādamātraḥ prakīrtitaḥ
sa ca dīrghassapādaḥ syādvarṇakālo nigadyate

An anusvāra following a long vowel is proclaimed to be a quarter-mora. / And that long vowel becomes one and a quarter morae; [thus] the timing of speech-sounds is stated.